Water-content-dependent carbonate formation in cemented paste backfill: phase assemblage, morphology, and ion partitioning

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Water controls ion transport during mineral carbonation, but its role is often conflated with direct evidence of phase-transformation mechanisms. Here, laboratory cemented paste backfill (CPB) was prepared from copper-mine tailings and a cementitious binder. The material was carbonated under five discrete water conditions: no added water at 55% relative humidity and liquid-to-solid ratios (L/S) of 0.4, 1, 3, and 20. Ex situ DTG, XRD, FTIR, SEM-EDS, ²⁹Si MAS NMR, pH, and ICP-OES were used to compare carbonate phase assemblage, morphology, silicate environments, and solution chemistry. Low-water samples retained broader carbonate signatures, detectable vaterite, and coating-like surface products. At higher L/S, calcite features became sharper and rhombohedral particles were more abundant. The carbonate-associated mass loss after 24 h varied from 13.37% to 14.25%, corresponding to apparent CaCO₃ contents of 30.4-32.4 wt% on a carbonated-product basis. Elemental maps and solution chemistry further indicated water-dependent partitioning of Ca and Mg. These ex situ observations are consistent with an increasing contribution from solution-mediated transport as water content rises. They do not directly establish a continuous amorphous calcium carbonate-to-vaterite-to-calcite pathway or quantify calcite purity. The tested L/S values therefore define composition-specific regimes rather than a universal threshold or engineering optimum. This work provides a bounded case study of moisture-dependent carbonation in one CPB formulation and identifies the quantitative measurements needed for process validation.